Ongoing advances in solid organ and hematopoietic stem cell transplantation (HSCT), including new immunosuppressive agents and improvements in histocompatibility matching, organ procurement, and surgical techniques, are gradually improving the outcome of clinical transplantation (Hariharan et al, 2000. N. Engl. J. Med. 342:605-12). However, chronic allograft rejection remains the prime determinant of long-term graft survival (Paul. L. C., 1999, Kidney International 56:783-793). Furthermore, stem cell graft rejection typically limits the application of allogeneic HSCT to those patients having an HLA-matched sibling donor, which represents a minority of all patients that might benefit from allogeneic HSCT therapy.
Tissue transplantation between genetically non-identical individuals results in immunological rejection of the tissue through T cell-dependent mechanisms. To prevent allograft rejection, immunosuppressive agents such as calcineurin phosphatase inhibitors and glucocorticosteroids which directly or indirectly interfere with IL-2 signaling are administered to transplant recipients (see, e.g., Morris, P. J., 1991, Curr. Opin. Immunol. 3:748-751; Sigal et al., 1992, Ann. Rev. Immunol. 10:519-560; and L'Azou et al., 1999, Arch. Toxicol. 73:337-345). The most commonly used immunosuppressive agents today are the calcineurin inhibitors cyclosporin A and FK506, which act relatively indiscriminately by impairing T cell receptor (“TCR”) signal transduction. A third primary immune suppression drug, rapamycin, which has recently received FDA approval for prevention of organ transplant rejection, acts through a distinct mechanism of inhibition of the protein mammalian target of rapamycin (mTOR). The biological effect of these three immunosuppressive agents is short-lasting, and as such, transplant recipients normally require life-long treatment of immunosuppressive agents to prevent transplant rejection. As a result of the long-term nonspecific immunosuppression, these immunosuppressive agents have many serious adverse effects. For example, the administration of cyclosporin A or FK506 to a transplant recipient results in degenerative changes in renal tubules. Transplant recipients receiving long-term immunosuppressive treatment have a high risk of developing infections and tumors. For example, patients receiving immunotherapy are at higher risk of developing lymphomas, skin tumors and brain tumors (see, e.g., Fellstrom et al., 1993, Immunol. Rev. 134:83-98).
In addition to graft rejection, immune T cells also mediate the primary cause of lethality after allogeneic HSCT, graft-versus-host disease (GVHD). GVHD, which is primarily initiated by donor CD4+ T cells expressing a Th1 cytokine phenotype characterized by IL-2 and IFN-γ secretion, manifests clinically as damage to the skin, intestine, liver, and immune system. To reduce the incidence and severity of GVHD, immune suppression therapy involving either cyclosporin A or FK506 is typically administered, often in combination with other immune suppression agents such as methotrexate. This immune suppression approach to the prevention of GVHD is problematic, as significant morbidity and mortality from GVHD still occurs, and the immune suppression therapy reduces the potency of the allogeneic T cell-mediated graft-versus-leukemia (GVL) or graft-versus-tumor (GVT) effect, and predisposes to multiple viral, bacterial, and fungal infections.
An alternative to immunosuppressive agents for the prevention of allograft rejection is the blockage of specific receptors involved in T cell costimulation. T cell activation requires both TCR-mediated signal transduction and simultaneously delivered costimulatory signals. These costimulatory signals are contributed, in part, by the activation of the costimulatory molecule CD28, which is expressed on resting T cells, by CD80 (B7-1) or CD86 (B7-2) expressed on antigen presenting cells (“APCs”). The activation of the costimulatory molecule CD40, which is expressed on APCs (i.e., B cells, dendritic cells, and macrophages), by CD40 ligand (“CD40L”), which is expressed on activated T cells, contributes to the upregulation of T cell activation by inducing the expression of B7-1 and B7-2 on APCs and the production of certain chemokines and cytokines such as IL-8, MIP-1-α, TNF-α, and IL-12 (Cella et al., 1996, J. Exp. Med. 184:747-752: and Caux et al., 1994, J. Exp. Med. 180:1263-1272). The CD40/CD40L interaction also results in the differentiation of T cells to T helper (“Th”) type 1 cells in part due to the expression of cytokines such as IL-12 by dendritic cells and macrophages.
CTLA-4 is normally expressed as a membrane-bound receptor on T cells and, similar to CD28, binds to B7-1 and B7-2 molecules on APCs; however, signaling of T cells via CTLA-4 downregulates T cells. The administration of soluble CTLA-4Ig is believed to prevent allograft rejection by competing with CD28 for B7-1 and B7-2. Soluble CTLA-4Ig has been administered to transplant recipients to disrupt the CD28/B7 interaction so that T cell costimulation is blocked and allograft rejection does not occur (Zheng et al., 1999, J. Immunol. 162:4983-4990; Lenschow et al., 1996, Ann. Rev. Immunol. 14:233-258). Unfortunately, CTLA-4Ig has variable efficacy, and typically does not prevent development of chronic rejection.
Anti-CD40L (anti-CD154) monoclonal antibodies have also been administered to transplant recipients to prevent allogaft rejection. These antibodies function by blocking the interaction of CD40 on antigen presenting cells (APC) and CD40L on activated T cells. It has recently been shown that graft survival achieved through the use of anti-CD40L monoclonal antibodies results in a significant inhibition of Th1 type cytokines (i.e., IL-2, IL-12; TNF-α, and IFN-α), and an increase in the levels of the Th2 type cytokines (i.e., IL-4, and IL-10) in the graft sections (Hancock et al., 1996, Proc. Natl. Acad. Sci. USA 93:13967-13972). Although the administration of anti-CD40L monoclonal antibodies has been shown to result in permanent graft survival when given to mice in combination with donor-specific spleen cells, adverse side effects such as coagulation have also been shown to be associated with the administration of anti-CD40L monoclonal antibodies. Initial clinical trials in adult renal transplant recipients receiving anti-CD40L monoclonal antibody plus glucocorticoids were halted because of thromboembolic complications though the extent to which thromboembolism was attributable to monoclonal antibodies versus non-specific factors in the antibody formulation is unclear (Kawai et al., 2000, Nature Med. 6:114; and Kirk et al., 2000, Nature Med. 6:114). Further, in the primate renal allograft study, concomitant use of mainstream immunosuppressive agents such as FK-506, methylprednisolone and mycophenolate mofetil diminished the efficacy of CD40L (CD154) mAb, though the exact contribution of each of the individual drugs to this reduction in efficacy was not determined (Kirk, A. D., 1999, Nature Medicine 5:686-693.).
Immunocompromised patients lack a fully active and effective immune system, and are vulnerable to infection by a host of opportunistic organisms that are effectively controlled in a healthy individual. Cancer patients and transplant recipients are especially vulnerable to these infections since their therapeutic regimen often includes radiation and chemotherapeutic agents, which compromise the immune system. Immunodeficient patients, such as AIDS and SCID patients, are also at high risk from these opportunistic pathogens. In particular, patients undergoing bone marrow transplantation (BMT) are severely immunocompromised until their immune systems reconstitute. During the period prior to reconstitution, these patients are susceptible to serious, and sometimes fatal, virus infections caused by normally benign viruses such as adenovirus, cytomegalovirus (CMV), and Epstein-Barr virus (EBV).
In a normal individual, recognition and destruction of virally infected cells is performed principally by CD8+ cytotoxic T lymphocytes (CTLs). The mounting of a CTL immune response requires that the viral proteins undergo intracellular processing to peptide fragments. Selected peptides of defined length are subsequently presented at the cell surface in conjunction with MHC class I molecules. This complex provides the first stimulatory signal recognized by the specific cytotoxic T lymphocyte.
Processing of antigens for presentation by class I MHC involves a complex cellular process (Berzofsky and Berkower, Fundamental Immunology, Third Edition, Paul (ed.), Raven Press, Ltd.: New York, pp. 258-259 (1993). Unlike processing of exogenous antigen via endosomal pathways for presentation by class II MHC, antigen presented by class I MHC generally must be synthesized endogenously and processed by a nonendosomal pathway into peptides. However, exogenous antigens can enter the cytoplasm for processing by the nonendosomal pathway and presentation by class I MHC.
No satisfactory methods presently exist for monitoring whether a transplant graft is being accepted or rejected by a recipient. In general, signs of cellular damage within the transplant tissue can be assayed. Alternatively, for tissues such as kidney or liver, physiological function of the transplant tissue can be assayed. Often, however, by the time overt signs of either cellular damage or a decrease in physiological function are detected, the tissue graft is already beyond rescue. This is particularly true in the case of such organ transplants as heart transplants, with which the first overt signs of rejection are often complete failure of the heart's function. Similarly, in the setting of allogeneic HSCT, there exist no reliable method to detect GVHD prior to the onset of significant end-organ impairment; oftentimes, when GVHD does develop, the donor immune reaction is relatively mature, and can thereby be refractory to even the most potent immune suppression therapies available.
Accordingly, there is a need for improved, safer immunomodulatory treatments that have long-lasting effects for the prevention of transplant rejection or GVHD. In particular, there is a need for treatments that are more specific and less toxic than the currently available therapeutic agents.
In addition to graft rejection and GVHD, immune T cells of autologous or allogeneic source may play a beneficial role in mediating anti-tumor effects and anti-infectious disease effects, including against viral, bacterial, and fungal processes. This T cell biology offers the possibility that adoptive transfer of ex vivo generated T cell populations might be utilized in the therapy of cancer or infection. However, full realization of this possibility is limited by a general inability to amplify a potent autologous immune response against cancer or infectious disease antigens in vivo. Furthermore, immune T cell therapy in the allogeneic setting is limited by allogeneic T cell attack against normal host tissues, which is manifested as GVHD. In the allogeneic anti-tumor immune therapy setting, the graft-versus-leukemia (GVL) or graft-versus-tumor (GVT) effect is reduced by the immune suppression drugs cyclosporine A, FK506, corticosteroids, and methotrexate that are utilized to prevent or treat GVHD. Avoidance of standard GVHD prevention or treatment agents through rapamycin administration post-transplant will predictably facilitate improved GVL and GVT effects, resulting in improved rates of cancer cure.